The PUREX process has become established in the reprocessing of irradiated nuclear fuels and reference can be made to the journal "Zeitschrift Atomkernenergie - Kerntechnik", volume 35, (1980) issue 2, pages 81 to 93. Page 87 of this issue discloses that organic solvents can be washed to remove the degradation products contained therein. The solvent should be passed in circulation and must therefore pass through this solvent washing. Soluble impurities and decomposition products are removed from the solvent by alkaline washing with sodium carbonate solution. In this connection, reference can be made to "Zeitschrift Atomwirtschaft - Atomtechnik", volume 26, Number 3, March 1981.
The PUREX process uses an organic solvent (preferably 30 volume percent TBP in dodecane), which is partly hydrolyzed by the contact with acid solutions and is partly decomposed radiolytically by the radioactive radiation during the reprocessing process. These acid decomposition products are washed out by a single-stage and/or multi-stage alkaline/acid washing before recycling of the solvent. Sodium carbonate (Na.sub.2 CO.sub.3) in the form of an aqueous solution is often used as the washing solution, and as spent washing solution chiefly contributes, after evaporation, to the salt load (NaNO.sub.3) of the moderately active waste (MAW) of a reprocessing facility. The alkaline washing solutions do not remain in use until they are neutralized, since otherwise certain metal complex compounds are hydrolyzed and precipitate.
Sodium carbonate solutions or sodium hydroxide solution have usually been chosen as the washing solution for the organic solvent, and have always been introduced in excess, assuming the most unfavorable conditions. It has been found here that only 10% of the washing agent is available for breaking down the degradation products in the organic solvent, since up to 90% is consumed by secondary reactions. These secondary reactions are caused by the entrained acids and heavy metals, such as uranium which are complexed by the sodium carbonate solution and remain in the solution.
A process for solvent washing in which the solvent is washed with an aqueous hydrazine hydrate solution with a molar concentration of the order of 0.1-1.0 is known from German patent publication DE 24 49 589 C2. The moderately active waste occurring as a result of the washing solution is thereby said to be reduced by a factor of 100. From the point of view of a reduction in the waste, the solvent washing is subjected to discontinuous checking.
The use of hydrazine hydrate solution has considerable disadvantages for the further processing of the hydrazine waste. An additional process is necessary before evaporation of the aqueous waste. The hydrazine must be destroyed by electrolytic oxidation. This electrolytic oxidation, however, can only be carried out if the hydrazine waste is first rendered strongly acid. The organic phase still present can thereby be separated out. In this solvent washing also, the hydrazine hydrate must be added in excess.
Such solvent washings are predominantly carried out in so-called mixer-settlers. Such mixer-settlers, which can be built up from one or more stages, are known from German patent publications DE-AS 26 24 936 and DE-PS 29 24 458. It is a characteristic feature of these mixer-settlers that each stage comprises a mixing chamber and a settling chamber in cascade therewith for separating the phases which have been mixed with one another.